It is only a "disorder" if you find this arousal intrusive and unwelcomed. If that is the case then you may have a rare sexual dysfunction called Persistent Genital Arousal Disorder or PGAD. My feeling though is that this is not what your are describing here and that you are perfectly normal.
Learn abdominal breathing. ( google it). Practice this breathing for 10 minutes twice a day. When you get unwanted arousal , take few abdominal breath and your sexual urge will disappear. This will also help you in many other ways to keep you healthy and smart.
Masturbating is okay. But watching pornographic film is not okay. Concentrate on sensation of your body minutely.This will make you aware of your body and prepare you for full body orgasm when you have intercourse. Sexual sensation and breathing have some connection. Learn some breathing exercise and practice regularly.